The cleanliness of electronic circuit assemblies (ECA'S), such as printed circuit board (PCB's) is critical to their proper function and reliability. Contamination leads, in many cases, to premature failure of the assemblies. Failures in this regard will mainly be due to the formation of shorts.
In the manufacture of electronic circuit assemblies, contamination can accumulate throughout the various steps of the fabrication process. One of the final steps in the fabrication process is the application of soldering flux, followed by various soldering operations. In practice, however, these fluxes have proven difficult to effectively remove.
In the fabrication of electronic circuit assemblies, soldering fluxes are applied to the substrate to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble fluxes. The rosin fluxes in general provide superior overall soldering performance as compared to the non-rosin fluxes. Thus, the rosin fluxes are widely used in the industry. Non-rosin or water soluble fluxes are generally inferior in soldering performance and tend to be more corrosive and therefore are not as widely utilized as the rosin fluxes.
One advantage of the non-rosin fluxes is their ability to be effectively cleaned using water based alkaline cleaners which have been known for years. According to U.S. Pat. No. 5,264,046 (Winston, et al), whose teachings are incorporated herein by reference in their entirety, these alkaline cleaners may also be utilized to clean rosin based fluxes, however, the ultimate cleanliness of the assemblies cleaned with Winston's process, is questionable.
Rosin based fluxes have traditionally been cleaned out most effectively with chlorinated hydrocarbon solvents, such as 1,1,1-trichloroethane, CFC 113 and CFC 112 or mixtures or azeotropes of these and other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment, deplete the ozone layer and/or contribute to the greenhouse effect. Thus, these solvents are subject to close scrutiny by the Occupational Safety and Health Administration and the Environmental Protection Agency.
In addition, alkaline cleaning compounds based on alkanolamines and/or caustic (U.S. Pat. No. 4,635,666) have been used in removing rosin fluxes. However, these processes have the distinct disadvantage of being significantly corrosive to solder and other materials of construction in the electronic assemblies. The related teachings of Winston, et al. have further specified the conditions of these processes by proposing particular alkaline solutions, which have been known for some time and which may be effective at flux removal.
In other approaches, Hayes et al., U.S. Pat. Nos. 4,640,719 and 4,740,247, rosin soldering flux and other residues are removed from electronic assemblies by means of terpene compounds in combination with terpene emulsifying surfactants by rinsing in water.
In addition to solder flux removal, the complete removal of adhesives and other residues also poses a problem. During the fabrication process, various tapes and/or adhesives may be utilized on a temporary basis. Once such protection is no longer needed, the adhesive residues must be removed. Removal of these adhesive residues has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the present invention addresses a method for effectively removing various residual contaminates, which are likely to be found on electronic circuit assemblies, including, without limitation, rosin flux, photoresist residue, soldermask residue, adhesive residues, oils, greases, silicones, lanolin, mold release, non-rosin fluxes, polyglycols, plasticizers and other problematic residues.